The invention is of use in producing visible images but is not limited to that field. In the context of the invention, the term imaging should be understood in a broad sense. It is for example possible to produce mappings of pressure or of temperature or even two-dimensional representations of chemical or electrical potentials. These mappings or representations form images of physical quantities.
In a detector, a pixel represents the basic sensitive element of the detector. Each pixel converts a physical phenomenon to which it is subjected to an electrical signal. The electrical signals from the different pixels are collected in a matrix reading phase then digitized so as to be able to be processed and stored to form an image. The pixels are formed by a zone sensitive to the physical phenomenon and delivering a current of electrical charges. The physical phenomenon can be an electromagnetic radiation and, subsequently, the invention will be explained by means of this type of radiation and the charge current is a function of the flow of photons received by the sensitive zone. Generalization to any imaging device will be easy.
The photosensitive zone generally comprises a photosensitive element, or photo detector, which can for example be a photodiode, two diodes head-to-tail, a photo resistor or a photo transistor. There are photosensitive matrices of large dimensions which can have several millions of pixels. Each pixel consists of a photosensitive element and an electronic circuit consisting, for example, of switches, capacitances, resistors, downstream of which there is an actuator. The assembly consisting of the photosensitive element and the electronic circuit makes it possible to generate electrical charges and collect them. The electronic circuit generally makes it possible to reset the charge collected in each pixel after a charge transfer. The role of the actuator is to transfer or copy the charges collected by the circuit into a column conductor. This transfer is performed when the actuator receives the instruction to do so. The output of the actuator corresponds to the output of the pixel.
In this type of detector, a pixel operates according to two phases: an image capture phase, during which the electronic circuit of the pixel accumulates the electrical charges generated by the photosensitive element, and a reading phase, during which the collected charges are transferred or copied into the column conductor by virtue of the actuator.
During the image capture phase, the actuator is passive and the collected electrical charges cause the potential to change at a point of connection between the photosensitive element and the actuator. This point of connection is called charge collection node of the pixel or, more simply, node of the pixel. During the reading phase, the actuator is active in order to release the charges accumulated at the photosensitive point in order to convey them or copy them, even copy the potential of the node of the pixel to a reading circuit of the detector arranged downstream of the actuator.
A passive actuator should be understood to be an actuator that is not in electrical contact with the reading circuit. Thus, when the actuator is passive, the charges collected in the pixel are neither transferred nor copied into the reading circuit.
An actuator can be a switch controlled by a clock signal. It is generally a transistor. It can also be a follower circuit or any other device making it possible to carry or transfer the charge collected in the pixel to the reading circuit, for example a device known by the acronym CTIA (Capacitive Translmpedance Amplifier).
This type of pixel can be used for the imaging of ionizing radiations, and notably X or y radiation detectors, in the medical field or that of non-destructive inspection in the industrial field, for the detection of radiological images. In some detectors, the photosensitive elements make it possible to detect a visible or near-visible electromagnetic radiation. These elements have little or no sensitivity to the incident radiation on the detector that is to be detected. A radiation converter, called scintillator, is then used which converts the incident radiation, for example an X radiation, into a radiation in a band of wavelengths to which the photosensitive elements present in the pixels are sensitive.
According to another type of detector, increasingly widely used, the detector material is a semiconductor, sensitive to the radiation, for example X or y, to be detected. An interaction of a radiation in the detector generates charge carriers. The charges generated are collected by an interaction at a terminal, called node of the pixel.
During the image capture phase, the electromagnetic radiation, in the form of photons received by each photosensitive element, is converted into electrical charges (electron/hole pairs) and each pixel generally comprises a capacitance making it possible to accumulate these charges to make the voltage of the node of the pixel change. This capacitance can be intrinsic to the photosensitive element, in which case it is called stray capacitance, or added in the form of a capacitor connected in parallel to the photosensitive element.
Generally, the pixels are read individually. The matrix can for example comprise a column conductor associated with each column of pixels of the matrix. In this case, a read instruction is sent to all the actuators of a same row of the matrix and each of the pixels of that row is read by transferring its electrical information, charge, voltage, current, frequency, etc., to the column conductor with which it is associated.
A number of pixels may be required to be grouped together so that they can be read collectively. This grouping can be useful in order to increase the reading speed of the matrix or even to improve the signal-to-noise ratio of each element read. The grouped pixels can have means for performing the operations of summing or averaging the electrical information items from the grouped pixels. These means can be analog or digital.
Subsequently, the case will be described in which the electrical information item is available in analog form in the pixels, in the form of quantities of charges stored on capacitors of the same value. It is of course understood that the invention can be implemented for any form of electrical information item generated in each of the pixels.
An image detector generally comprises an image sensor. The image sensor comprises row conductors, each linking the pixels of a same row, and column conductors, each linking the pixels of a same column. The column conductors are connected to a reading circuit, also called column reading block.
The connection between the column reading block and a column conductor is made at a bump contact. There are therefore, in the traditional configuration of an image detector, as many bump contacts as there are column conductors and therefore as there are columns of pixels. A spacing between two column conductors is defined. The spacing between two column conductors corresponds to the spacing between two bump contacts. For some applications, there may be a requirement to reduce the spacing between two column conductors while maintaining a greater spacing between two bump contacts. Such is the case notably in the field of medical imaging by X rays and more particularly for an application to mammography in which high quality images are required. The pixel can occupy a squared surface area of the order of 50 micrometers (μm) side, that is to say smaller than the usual sizes of pixels used in standard X radiology. The spacing between the bump contacts is then reduced. The connection of the bump contacts with the column reading block is made by means of connector modules. Now, the connector modules are not necessarily suited to small spacings between two bump contacts.